STUCK ON DOWNGRADING 3.50 PLEASE HELP!!!!

i am currently trying to downgrade my 3.50 fw psp, i now have 3.50 hen but everytime i click on the x.yz update it says "cannot find update.pbp." i have tried it mulitiple times, withj different 1.50 eboots, and i do have a copy of luminies. SOMONE PLEASE HELP!!!